The present invention relates to an image sensor used in an image input device of a facsimile machine, scanner, etc. More specifically, the invention relates to a TFT-drive-type image sensor in which linearly arranged photodetectors are divided into a plurality of blocks and thin-film transistors are connected to the respective photodetectors to read their output signals on a block-by-block basis by matrix-type driving.
In a conventional contact-type image sensor, light reflected from a document surface is input, via a rod lens array, to a long photodetector array whose length is approximately equal to a document width and an electrical signal corresponding to image information on the document is produced by photoelectric conversion from respective photodetectors constituting the photodetector array.
In a TFT-drive-type image sensor, which has been proposed as the above type of image sensor, charges generated in the respective photodetectors are transferred on a block basis by use of thin-film transistors (TFT) and a matrix wiring and temporarily stored in storage capacitors. Then, a detection circuit reads out signals in time sequence on a block basis. Thus, it is possible to read one-line image information by a single driver IC, to thereby reduce the production cost.
FIG. 6 shows an example of the TFT-drive-type image sensor which includes a photodetector array 50 in which a plurality of photodetectors P are arranged at a certain density over a length approximately equal to a document width, a thin-film transistor array 51 consisting of a plurality of thin-film transistors T that are in one-to-one correspondence with the photodetectors p, a driver IC 52 for detecting charges generated in the photodetectors P, and a wiring 53 for connecting the thin-film transistors T and the driver IC 52 in matrix form. The photodetector array 50 consists of K blocks of photodetectors p and each block contains n photodetectors P. Each photodetector P is connected to the drain of the corresponding thin-film transistor T, and the sources of the n thin-film transistors T of one block are connected individually to the driver IC 52. The gates of the thin-film transistors T are connected to gate drive lines G.sub.1 -G.sub.K on a block basis.
Each photodetector P is a photodiode in which a positive voltage V.sub.B is applied to its cathode to establish a reverse-biased condition. With reflection light from a document surface incident on the photodetector array 50, electron-hole pairs are generated during a storage period and charges are stored in an equivalent capacitance of the photodetector P and an overlap capacitance between the gate and drain of the thin-film transistor T. When a pulse is applied to the gate drive line G.sub.1, the thin-film transistors T.sub.1,1 -T.sub.l,n turn conductive and charges of n bits on the drain side of those thin-film transistors are transferred to capacitances C.sub.L of the wiring. The charges stored in the capacitances C.sub.L cause potential variations of respective common signal lines 54, which are connected to the driver IC 52. The varied potentials are detected by voltage follower amplifiers of the driver IC 52, and then supplied to an output line 55 in time sequence by an analog multiplexer. The similar operation is repeated thereafter; that is, pulses are applied to the gate drive lines G.sub.2 -G.sub.k to sequentially turn on the thin-film transistors T on a block basis. As a result, signals of nxK bits are read out in time sequence. Further, the above operation is repeated while the document is moved by a document feeding means (not shown) such as rollers, to obtain an image signal for the entire document. Reference is made to Japanese Patent Application Unexamined Publication No. Sho. 63-9358.
Amorphous silicon (a-Si) is used as the semiconductor material of the thin-film transistors T of the TFT-drive-type image sensor, because a number of thin-film transistors T need to be formed on a substrate of a large area. However, amorphous silicon as the semiconductor material essentially has many traps. Therefore, when the thin-film transistor T operates as a switching device, charges released from the traps flow to the source side during an off state. This is observed as a phenomenon similar to the off leak current.
More specifically, when a pulse is applied to the gate of the thin-film transistor T (see FIG. 7(a)), charges flow between its drain and source during a charge transfer (see FIG. 7(b)). Due to the existence of the traps, charges continue to flow at a large time constant even after the transistor T is turned off. After the charges of the image signal generated in the photodetector P have been transferred to the wiring capacitance C.sub.L, the charges released from the traps are also stored in the wiring capacitance C.sub.L irrespective of whether the transferred charges are a light output or a dark output. Therefore, when the potential of the common signal line 54 as changed by the charges stored in the wiring capacitance C.sub.L is read out, unnecessary stored charges of .DELTA.q are superimposed, as an offset, on the signal, to deteriorate the gradation reproduction performance of the image sensor. Further, since the amount of .DELTA.q depends on the areas of the source, drain and gate electrodes or the area of the channel region, the dark output varies pixel by pixel.